Liquid-crystal (LC) cells can change the polarization of light traveling across the cells under control of an electrical signal. Thus, in conjunction with polarization-selective optical components, LC cells can be used to build optical devices, such as displays, optical switches, optical multiplexers and electrically controllable optical attenuators.
The polarization changing property of LC cells is dependent on the root-mean-square (RMS) amplitude of an applied AC electric field. Typically, the magnitude of polarization change is a continuous function of the RMS amplitude of the driving voltage. For many applications, notably in optical communications applications, very precise control over electrical driving conditions of the LC cells and long-term stability are required. To meet these requirements, it is necessary to generate driving signals with constant RMS voltage. One efficient method to adjust the RMS voltage consists of varying the duty cycle of a binary driving signal.
However, rapid transitions in the driving signal may lead to problems. If the driving circuit generates very sharp transitions, the actual electric field experienced by the LC cells will be low-pass filtered as a result of electrode resistance and the inherent capacitance of the cells. Electrode resistance is typically substantial because most LC applications require at least one of the electrodes to be transparent, which prohibits the use of material having a high electrical conductivity for the electrodes. In addition, the inherent capacitance of an LC cell depends on the orientation of the LC molecules within the material, and is therefore, bound to change over time. As a result, the effective RMS voltage experienced by the LC cells is significantly dependent on factors outside the control of the driver circuit, such as the parasitic resistance and capacitance of the LC cells. In addition to this problem, some LC materials exhibit substantial dependency on the frequency of the driving signal, whereby higher harmonics may effectively counteract the action of the fundamental driving component.
In view of these concerns, there is a need for a driver circuit and method for driving an electrical device, such as a LC device, with high accuracy and long-term stability.